In micro-machining, a number of different material transformations exist which allow a laser beam to remove or machine material: ionization, melting/boiling, photo-chemical material modification and photo-chemical reaction with process gasses (for example). Material removal is typically accompanied by a plasma plume rising from the surface of the machined regions after a laser pulse is delivered. The duration of the plasma plume is short (typically 1 to 10 microseconds) but it possesses a very high temperature and the ions produced have high velocities.
Through a combination of conduction, convection and radiative transfer of energy from the region of the laser beam's interaction with the machining material and from the plasma plume arising from the decomposition products of the machined material a Heat Affected Zone (HAZ) is created. The heat energy in the HAZ can alter the material properties of the un-machined material in an undesirable manner.
The energy contained in the plasma plume is dependent on raw material composition, laser fluence and the contiguous area of the machining site. Laser fluence is typically set to optimize parameters such as machined edge quality, material removal rate, debris formation and other attributes on the finished part. Accordingly, for a given fluence, the volume of the plasma plume is dependent on the total contiguous area of the machined site. Accordingly, the larger the area, the larger the plasma plume volume. Moreover, the volume of the plasma plume determines the magnitude of localized heating (i.e., the HAZ) of the perimeter of the machining site.
Larger plumes do not get cooled or quenched as quickly as smaller plumes since there is proportionately less surface area exposed to the air relative to the volume of the plume. For example, FIG. 1A illustrates a relatively large contiguous area being machined and an associated large plasma plume, while FIG. 1B illustrates a small plume associated with a smaller machined area. Thus, the smaller plume will be cooled/quenched quicker since a larger fraction of the plasma plume surface area is in contact with air.
For many materials undergoing pulsed laser micro-machining, the maximum laser pulse repetition rate is limited to a rate that produces an acceptable Heat Affected Zone (HAZ). When the laser pulse rate is too high, the magnitude and rate of plasma formation cause heating of un-machined material around the machined area causing an unacceptable HAZ. Although it is possible to reduce the laser pulse rate to allow material cooling between pulses, such a reduction results in a direct decrease in the throughput of the machining process. Thus, throughput of laser micro-machining could be increased if the one were able to limit the volume of the plume associated with the machined area.